/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/PBDRigidClustering.h
Manually add a set of particles to a cluster after the cluster has already been created. ChildToParentMap: A map that may contain a pointer to one of the child particles as a key, and a pointer to its old parent particle (prior to be being released for example). If a child particle exists in this map, its parent (or whatever is specified as the value) will be used to determine the correct proxy to add to the parent cluster.
void AddParticlesToCluster ( [FPBDRigidClusteredParticleHandle](API\Runtime\Chaos\Chaos\FPBDRigidClusteredParticleHandle) * Cluster, const [TArray](API\Runtime\Core\Containers\TArray)< [FPBDRigidParticleHandle](API\Runtime\Chaos\Chaos\FPBDRigidParticleHandle) * > & InChildren, const [TMap](API\Runtime\Core\Containers\TMap)< [FPBDRigidParticleHandle](API\Runtime\Chaos\Chaos\FPBDRigidParticleHandle) *, [FPBDRigidParticleHandle](API\Runtime\Chaos\Chaos\FPBDRigidParticleHandle) * > & ChildToParentMap )